Clamping devices of the above type are well known in the art. Such devices have sample holding elements with spherical outer surfaces which are received in spherical segment recesses of various types of clamping devices. Reference is made in this connection, for example, to German Patent Publication (DE-AS) 2,028,030, published on Nov. 30, 1972, and disclosing a clamping mechanism in which clamping jaws (2) are supported between sickle-shaped wedges (3) which in turn are slideable and adjustable between two clamping pressure applying elements (10) which are mounted on a base plate (9). The clamping elements (10) apply a sufficient pressure to hold the respective end of the test samples in such a way that the testing force or load can be applied to the test sample through a friction contact between the end of the test sample and the clamping jaws. Such a structure is relatively complicated and leaves room for improvement, especially with regard to avoiding the application of bending moments to the test sample.
In connection with testing samples for tensile and/or compresssion strength, it is necessary that exactly defined loads can be applied to the sample being tested. It is especially important that the test sample is not exposed to bending loads which can occur unintentionally, for example, if the test sample is canting, or when the cooperating surfaces of the clamping mechanism are mismatched, for example, due to imprecisions in the manufacturing process, especially at the points where the sample contacts the clamping jaws. Avoiding the application of bending moments to the test sample is especially important in connection with ceramic test samples which are particularly sensitive to bending loads, since ceramics are brittle.
In addition to the above mentioned German Patent Publication (DE-AS) 2,028,030, reference is made to German Patent (DE-PS) 671,954, and German Publication (DE-AS) 2,306,393. These publications illustrate efforts to avoid the application of bending moments to a test sample in a clamping mechanism of a testing machine. Both known clamping mechanisms employ cooperating spherical surfaces. However, it has been found that the known clamping mechanisms are not suitable for testing sensitive components, for example, of ceramic materials which are be tested by tensile forces and/or compression forces exclusively. The known devices either permit only the application of tensile forces as is the case with the just mentioned two publications or a complicated structure is required as is the case in the first mentioned publication (DE-AS) 2,028,030.
U.S. Pat. No. 1,341,431 (Morrow) discloses a grip for testing samples in which conical or spherical surface areas cooperate with each other. Morrow also wants to avoid the application of "lateral stress" to the test sample. Compression forces cannot be applied in the Morrow apparatus. The same considerations apply to the gripping device of U.S. Pat. No. 2,896,448 (Haines) in which, for example, sheet metal to be tested is wound partially around a split pin which in turn is held in a cylindrical hole. Haines wants to distribute the applied testing stress evenly over the entire cross-section of the specimen. However, it is not certain that bending stress can be avoided because the clamping mechanism directly contacts the sides of the specimen.
German Patent Publication (DE-OS) 3,316,218 discloses a mechanism for applying compressive loads to a test sample in a material testing apparatus whereby the cooperating surfaces are also spherical to assure a central introduction of the compressive testing force into the test sample.
East German Patent Publication 71,217, published on Nov. 5, 1970 discloses a self-centering clamping mechanism for test samples in which a conical surface of the clamp body cooperates with a movable spherical surface. A ring contact is established in this manner to assure the self centering.
East German Patent Publication 250,376, published on Oct. 8, 1987 discloses a clamping mechanism, especially suitable for testing sectional steel members, specifically the strength of welding seams between such sectional steel members. Suggestions toward the present combination are not found in the above discussed references.